I used to work as a mechanic apprentice in my village and I know of three reasons , I am sure there are more reasons.
it might be that the valve seals are bad, there purpose is to prvent oil ( who lubricates the valve-train ) from flowing down into the combustion chamber and to be burn. Some times it can be the cylindrical chamber ( called a valve guide ), that are worn , in this case again you will get oil flowing to the combustion chamber.
another thing can be high pressuer of carbon gas, which pressurizes the oil pan and pushes oil up into the fuel delivery system, where it is fed into the engine and burned. Or worn piston rings
To avoid these visit your mechanic from time to time and use quality products